A typical optical display system contains a light source that is required to observe the information presented by the display. In battery powered equipment like laptop computers, the light source can represent a substantial fraction of the total power draw of the equipment. Therefore, reducing the amount of power required to produce a given luminance can increase battery life, which may be especially desirable in battery powered equipment.
The 3M brand Brightness Enhancement Film (BEF) available from the 3M Company of St. Paul, Minn., is one type of optical film that can be used to address this problem. BEF and similar films typically include an array of prisms on one surface that are large in comparison with the wavelengths of light. The structures can increase on-axis brightness of optical display systems by redirecting off-axis light and recycling the on-axis light such that it eventually emerges from the display on-axis, i.e., generally directed toward a viewer. In use, these films typically increase on-axis luminance at the expense of off-axis luminance in one or two dimensions. In this fashion, the material can help the display designer achieve the desired amount of on-axis luminance with reduced power consumption. Various embodiments of BEF and other similar films having structured surfaces are described in, e.g., U.S. Pat. Nos. 5,394,255 (Yokota et al.); 5,552,907 (Yokota et al.); 5,917,664 (O'Neill et al.); 6,052,164 (Cobb, Jr. et al.); 6,091,547 (Gardiner et al.); and 6,111,696 (Allen et al.).
Brightness enhancement is achieved with such prismatic structured surface films through a process of reflection and refraction that preferably includes light recycling. When used in preferred backlight configurations, prismatic structured surface films transmit light in the direction of the viewer (usually directly in front of the LCD) that would otherwise leave the screen at a high angle (missing the viewer).
In order for a prismatic structured surface film to direct light into a narrower angular exit profile toward the user, the film often includes a planar or nearly planar entry surface (on the opposite side of the film from the prisms) that includes an interface with air or another material with a sufficiently low index of refraction. The entry surface generally prohibits light from entering the film at internal angles greater than about 40 degrees from a normal direction defined by the entry surface.
To operate more efficiently, the entry surface of the prismatic structured surface film is typically separated by an air gap from other components in the display system so that the refraction characteristics of light entering the prismatic structured surface film through its entry surface are not affected by the generally higher indices of refraction of, e.g., a diffusing layer, light guide, etc. For example, air has an index of refraction of one (1), while polymers used to construct the diffusing layers typically have a higher index of refraction that is closer to the index of refraction of the materials used to manufacture the prismatic structured surface film. As a result, light refraction at a polymer interface is different than refraction of light at an air interface. Current techniques for manufacturing optical display systems that include prismatic structured surface films with entry surfaces commonly rely on the air gaps that will inherently exist between two films layered with each other in the absence of an adhesive or other agent that would optically couple the two layers.
This approach can, however, increase the cost of assembling the optical display because of the need to assemble the prismatic structured surface film with the other components in the optical display system. The layering approach may also lead to inconsistencies in appearance of the optical display as the air gap between the entry surface of the prismatic structured surface film and other components adjacent the entry surface varies over the entry surface. For example, in some areas the entry surface may be in contact with an adjacent component in a manner that negatively affects the refraction of light into film at the entry surface.